Flatten Binary Tree to Linked List

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
        TreeNode cur = root;
        while(cur!=null){
            if(cur.left!=null){
                TreeNode r = cur.left;
                while(r.right!=null) r = r.right;
                r.right = cur.right;
                cur.right = cur.left;
                cur.left = null;
            }
            cur = cur.right;
        }
    }
}